Method of and apparatus for compensating for residual magnetization in cathode-ray apparatus



Oct. 25, 1949. F. T. COUGHLIN 2,485,569 METHOD OF AND APPARATUS FORCOMPENSATING FOR RESIDUAL MAGNETIZATION IN CATHQDE-RAY APPARATUS FiledMarch 15, 1945 OSCILLOSCOPE SIGNAL SOURCE gwdq/wbcw FRANCIS T. COUGHLINI Patented Oct. 25, 1949 METHOD OF AND APPARATUS FOR COMPEN- SATING FORRESIDUAL MAGNETIZATION IN CATHODE-RAY APPARATUS Francis T. Coughlin,Washington, D. C.

Application March 15, 1945, Serial No. 582,962

1 Claim.

(Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 O. G. 757) This invention relates to amethod of and apparatus for compensating for residual magnetization incathode ray apparatus wherein the beam deflecting force is provided byan electromagnetic field.

After an electro-magnetically deflected cathode ray apparatus has beenin operation for some time, the iron cores of the cathode ray tubesdeflection coils may pick up a certain amount of residual magnetization.The presence of such residual magnetization in an apparatus wherein thecathode ray beam deflecting force is provided by a rotatingelectromagnetic field may be indicated on the tube as a circle insteadof a point. This has the effect of distorting the pattern and formingnon-opposing bilateral indications. In order to remove the residualmagnetization, one practice has been to insert a small iron coredemagnetizing coil energized from a 60 cycle 110 volt source in the areaof the deflecting coils and then to slowly withdraw the demagnetizingcoil, repeating the process until the pattern on the cathode ray tubeshows no indication of residual magnetization. Such a procedure is verytimeconsuming and uncertain.

An object of the present invention is to provide an effective andefficient method of and apparatus for compensating for residualmagnetization in cathode ray apparatus wherein the beam defiecting forceis provided by an electromagnetic field.

In accordance with one embodiment of the present invention, in order toovercome the distortion caused by residual magnetization in anelectromagnetically deflected cathode ray apparatus, a steady current ofproper amplitude and direction may be caused to flow in each set ofdeflecting coils setting up thereby a magnetomotive force which willoppose and cancel the residual magnetic field.

Other objects and advantages of the present invention will be apparentfrom the following detailed description taken in conjunction with thesingle figure wherein is schematically illustrated anelectromagnetically deflected cathode ray apparatus having associatedtherewith a residual magnetization compensating circuit constructed inaccordance with this invention.

Referring to the drawing, an electromagnetically deflected cathode raytube 5 is there illustrated having four deflecting coils 6, 1, 8, and 9,respectively, disposed about its periphery. The coils 6 and 8 and thecoils 1 and 9 form pairs and are respectively connected through leads I0and II and H and I2 to an oscilloscope signal source I3. Differentialenergization of the coils will cause deflection of the cathode ray beam,as is well known in the art. However, when the coils are no longerenergized from the signal source I3, it has been found that the previousenergization of the coils, in some cases, may result in the presence ofsome residual magnetization and this magnetization may be suificient tocause undesirable deflection of the electron beam. Thereafter when asignal is applied to the coils from the signal source, the resultingdeflecting force will be aifected by the deflecting force produced bythe residual magnetization and thereby distort the pattern on thecathode ray tube.

In order to overcome the effects of residual magnetization, inaccordance with this invention, a steady current of proper amplitude anddirection is caused to flow in the deflecting coils to set up amagneto-motive force which will oppose and cancel the residualmagnetization. While such a current could be provided by a pair ofbatteries, a more convenient source is that illustrated in the drawingwherein an isolating transformer M has its input winding [5 connected toa suitable alternating current source [6 and is provided on its outputside with two identical windings fl and [8, whereby a voltage on theorder of to volts is obtained from each winding. The winding 11 isconnected through a rectifier tube IE, to a low pass filter circuit 20,while the winding I8 is connected through a rectifier tube 2! to a lowpass filter circuit 22. Filament voltage for the tubes l9 and 2| may beprovided from a small winding 23 associated with the power transformer[4.

The rectified and filtered current from the winding i! is applied to aresistance bridge network, two arms of which are formed by a pair ofresistors 24 and 25 and the other two arms of which are formed by apotentiometer resistance 26. Similarly the rectified and filteredcurrent from the winding [8 is applied to a second resistance bridgenetwork comprising a pair of resistors 21 and 28 and a potentiometerresistance 29. The resistors 24, 25, 21 and 28 are of the same value.

The first resistance bridge network is connected to the deflecting coilsl and 9 through a lead 30 which is connected to the potentiometer 25 anda lead 3| which is connected between the resistors 24 and 25. The secondresistance bridge network is connected to the deflecting coils S and 8through the lead 30 which is also connected to the potentiometer 29 anda lead 33 which is connected between the resistors 21 and 28. Byadjusting the potentiometers 26 and 29 it will be readily apparent thatthe bridge networks may be so balanced that no current will flow in thecoils or that current will flow in one or both pairs of coils, thedirection or sense of the current as well as the amount depending on theadjustment of the two potentiometers.

In the operation of this apparatus, when it is apparent that with nosignal being applied to the deflecting coils from the signal source thebeam is being deflected, it is necessary merely to adjust one or both ofthe potentiometers to cause the beam to return to its correct position.

While but one embodiment of this invention has been shown and described,it will be understood that many changes and modifications may be madetherein without departing from the present invention.

The invention described herein may be manufactured and used by or forthe Government of p In combination, a magnetic deflection yokecomprising orthogonal pairs of iron cored deflection coils, a signalsource for separately activating each pair for efiecting deflectingoperation by core magnetization adjustable direct current supply meansconnected only across each pair and operative to supply constantcurrents of independently adjustable magnitude and direction to eachpair to cancel residual magnetism resulting from signal source operationindependently of coil potential.

FRANCIS T. COUGHLIN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 410,276 Haenichen Sept. 3, 18891,252,312 Warren Jan. 1, 1918 2,077,574 Malofi Apr. 20, 1937 2,086,926Stocker July 13, 1937 2,118,977 Lewis et a1. May 31, 1938 2,207,392Zuschlag July 9, 1940 2,223,990 Holmes Dec. 3, 1940 2,228,821 HansenJan. 14, 1941 2,280,733 Tolson Apr. 21, 1942 2,303,723 Claytor Dec. 1,1942 2,320,551 Bahring June 1, 1943 2,370,426 Schade Feb. 27, 19452,395,966 Goldberg Mar. 5, 1946

